| 21. | Two land masses collided at the end of the Ordovician Period about 466 million years ago.
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| 22. | These are from the Ordovician period and consist of chert, phyllite and quartz rich micaceous sandstone.
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| 23. | During the early and middle Ordovician period ( 485 to 460 Mya ), volcanic activity increased.
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| 24. | By the Ordovician period the gastropods were a varied group present in a range of aquatic habitats.
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| 25. | The valley lends its name to the Borrowdale Volcanic Group, a geological development from the Ordovician period.
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| 26. | These deposits occurred during the Ordovician period.
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| 27. | The "'Fish Haven Dolomite "'is an Ordovician period geologic formation in southern Idaho, northeastern Nevada, and northwestern Utah.
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| 28. | These fast-moving low-level epifaunal deposit feeders lived in the Cambrian and Ordovician periods, from 494.0 to 478.6 Ma.
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| 29. | These fast-moving low-level epifaunal deposit feeders lived in the Cambrian and Ordovician periods, from 498.5 to 478.6 Ma.
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| 30. | Graptolites with relatively few branches were derived from the dendroid graptolites at the beginning of the Ordovician period.
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